1. Field of the Invention
The present invention relates to rare earth magnets and a method of producing the rare earth magnets in which the main phase is R.sub.2 Fe.sub.14 B, where R is at least one rare earth selected from neodymium and praseodymium.
2. Description of the Prior Art
Rapid-quenched ribbons with good magnetic properties can be obtained by using a single-roll technique to rapidly cool an alloy melt containing rare earth R and representative transition metallic elements iron and boron in a ratio of substantially 2:14:1 (U.S. Pat. No. 4 756 775). Ribbons about 30 .mu.m thick are obtained by a single-roll rapid-quenching technique in which the melt of a Nd-Fe-B system alloy is ejected onto the peripheral surface of a rotating copper roll. The cooling conditions can be varied to achieve ribbon with a fine-grained microstructure with a grain size of 0.01 to 0.5 .mu.m.
The rapid-quenched Nd-Fe-B alloy thus obtained can then be ground into powder and consolidated nearly to full density by hot-pressing. This is reported in U.S. Pat. No. 4 792 367, JP-A-60-100402, and "Hot-pressed neodymium-iron-boron magnets" by R. W. Lee (Applied Physics Letters, vol. 46, No. 8, pp 790-791, Apr. 15, 1985). The hot-pressed bodies thus formed have yielded a residual magnetic flux density of around 8 kG.
To obtain a higher residual magnetic flux density it is necessary to induce anisotropy in the magnets. In his paper Lee proposed the use of plastic deformation to induce anisotropy. In this method hot-pressing is used to consolidate Nd-Fe-B powder to almost full density, and die-upsetting is then used to achieve plastic deformation of the pressed body. With this method, residual magnetic flux densities of 8 to 13 kG have been reported (for example, by Nozawa et al in J. Appl. Phys., Vol. 64, No. 10, pp 5285-5289, Nov. 15, 1988), depending on die-upsetting conditions and the composition of the alloy.
While magnets with high coercive force can thus be obtained by hot-deformation, a problem is that it involves a lengthy manufacturing process and factors such as surface cracking occurring during the plastic deformation make it difficult to form the magnets into product shapes.
Anisotropic sintered magnets are produced by grinding alloy ingots to obtain powder having a particle size smaller than the grain size, for example 3 .mu.m. The powder is then aligned in a magnetic field, cold-pressed and sintered. This method has provided Nd-Fe-B sintered magnets with good magnetic properties (cf JP-B-61-34242). However, the fact that this method involves the handling of highly active fine powder presents manufacturing problems. Also, the sintering is a conventional atmospheric pressure process that can give rise to dimensional changes and shape deformation, making it necessary to apply some post-machining to achieve the requisite product shape.